drm/amd/display: Increase precision for bpp in DSC calculations

[Why?]
Many DSC variables and related functions use whole bits for bpp.

[How?]
Change variables and related functions to use 16ths of a bit for bpp.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Dillon Varone <dillon.varone@amd.com>
Signed-off-by: Jun Lei <jun.lei@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>